1. Field of the Invention
The present invention relates generally to methods of acoustic measurements and determinations of subsurface parameters in a wellbore and its surrounding formation.
2. Background Art
The oil and gas industry uses various tools to probe the formation penetrated by a borehole in order to locate hydrocarbon reservoirs and to determine the types and quantities of hydrocarbons. For example, resistivity tools may be used to measure the conductivity of the formations, and sonic tools may be used to provide valuable information regarding formation mechanics. Such measurements can be performed with wireline tools, logging-while-drilling (LWD) tools, measurement-while-drilling (MWD) tools, or logging-while-tripping (LWT) tools. In this description, the difference between LWD and MWD is not germane. Therefore, LWD will be used herein to include both LWD and MWD tools or measurements.
LWD tools have become popular in recent years due to their ability to measure fresh formations shortly after a wellbore is drilled. In LWD logging, one or more tools are included in a bottom-hole assembly (BHA) that is disposed right behind the drill bit on the lower end of a drill string. The LWD tool takes measurements while a drilling operation is in progress. While LWD can provide formation measurements before the near wellbore region is significantly contaminated by the drilling fluids, it has its downside. Various techniques used in formation logging are sensitive to the mud flow, weight-on-bit, motion of the drill string, etc. Such undesirable interference can degrade the quality of the LWD data. For example, for LWD sonic measurements, the biggest factor affecting the quality of the logs is the drilling related acoustic noise.
In a typical drilling operation, sections of drill pipes have to be added from time to time to the existing drill string so that the drill bit can reach deeper formations. When addition of drill pipes is needed, the drilling fluid pump is stopped and the drill string is more or less stationary in the bottom hole during these periods, which are referred to as pump-off periods. Pump-off periods are also needed in certain operations, such as fishing operations.
During the pump-off periods, the LWD tools typically continue the measurements because they are programmed to take measurements at preset time intervals before the tool is lowered into a borehole. However, such measurement data are typically discarded because the tool is stationary at a specific depth where measurements have been taken before the pump is shut off. In addition, measurements acquired during the pump-off periods occur when mud telemetry is not available.
Data collection during pump-off periods is disclosed in U.S. Pat. No. 6,237,404 issued to Crary et al. and assigned to the present assignee. This patent is incorporated by reference in its entirety. This patent discloses methods that make use of the pump-off periods to obtain formation measurements that take a long time or that benefit from a quiet environment. Measurements acquired during pump-off periods may contain high-quality data that are otherwise unavailable. Therefore, it is desirable to have methods that make use of such high-quality data.